Stents are endoluminal devices that are inserted into body lumens and expanded in order to maintain the patency of the lumen. It is known for example, to use a stent to maintain the patency of an artery, a urethra, or a gastrointestinal organ.
A stent is essentially a cylindrical device that can exist in two conformations. In the small caliber conformation, the stent is inserted into the body and delivered to the lumen to be treated. Once correctly positioned in the lumen, the stent is deployed by being brought into a large caliber in which it applies radially outward forces against the inner wall of the lumen. The stent is constructed so as to be able to withstand all radially inward forces applied to it by the lumen wall, so that the caliber of the stent does not change after deployment in the lumen.
A stent may be formed, for example, from an elastic material that is unstrained when the stent is in the large caliber conformation. The stent is then mechanically constrained to bring it into the small caliber conformation. This stent may be constrained in the small caliber conformation by inserting it into a restraining sleeve. After positioning in the body, the restraining sleeve is removed. Due to the elastic properties of the stent, the stent spontaneously transforms into the large caliber conformation.
It is also known to form a stent from a material that becomes plastic when strained. The stent is formed in the small caliber conformation in which it is unstrained. A balloon is inserted into the lumen of the stent. The stent is then positioned in the body and the balloon is inflated. This expands the stent into the large caliber conformation by causing a plastic deformation of the stent material.
It is further known to form a stent from a shape memory alloy, such as Nitinol™. A shape memory alloy may exist in two states: a state in which it is super-elastic (the austenitic state) and a state in which it is soft (the martensitic state). The alloy in the austenitic state is formed into a stent in its large caliber conformation. The alloy is then brought into the martensitic state either by cooling the alloy or straining it. In the martensitic state, the alloy is deformed into the small caliber conformation in which it is delivered to the lumen to be treated. After positioning, the alloy is brought into the austenitic state by heating the alloy. In the austenitic state, the stent regains the large caliber conformation due to the shape-memory properties of the alloy.
It is further known to form a stent from a biostable or biodegradable elastic shape memory polymer. The shape memory capability of these polymers allows stents made of these materials to be inserted through small openings and then enlarging their caliber by an increase in temperature. The shape memory effect of polymers is a physical property exhibited best by amorphous polymers whose glass transition temperature is marginally higher than room temperature and whose transition from glass to rubber is particularly sharp. In this case, strain energy can be stored in the polymer by mechanical deformation (e.g. by stretching) followed by cooling. Recovery of the shape memory is exhibited upon reheating the material above the temperature to which it was cooled, allowing a return of the stretched polymer chains to more equilibrium, coiled structures.
There are many body lumens whose caliber fluctuates dynamically over time. Such body lumens include, for example, a pulsating artery, a peristaltic digestive tract organ or a portion of a lumen adjacent to a sphincter.
A sphincter is a formation of muscle tissue encircling a portion of a body lumen. Contraction of the sphincter occludes the lumen to prevent the passage of material in the lumen from one side of the sphincter to the other. For example, the gastro-esophageal sphincter is located at the junction between the esophagus and the stomach. When closed it prevents the stomach contents from refluxing into the esophagus. The pyloric sphincter is located at the junction between the stomach and the small intestine. When closed, it prevents the stomach contents from entering the small intestine. The voluntary urethral sphincter is located below the prostatic urethra and controls the flow of urine from the urinary bladder.
It is often desired to place a stent in a body lumen whose diameter changes dynamically over time. For example, it may be desired to place a stent in a region of a body lumen adjacent to a sphincter. Thus, it may be desired to place a stent in the prostatic urethra near the voluntary urethral sphincter. It may also be desired to place a stent in the esophagus adjacent to the gastro-esophageal sphincter, to place a stent in the duodenum adjacent to the pyloric sphincter, or to place a stent in the common bile duct adjacent to the Oddi sphincter. Placing a cylindrical stent in a body lumen adjacent to a sphincter often interferes with the proper functioning of the sphincter. The radially outward forces exerted on the inner wall of the lumen near the sphincter by the stent may prevent the lumen from becoming completely occluded when the sphincter contracts.
A pulsating blood vessel is another example of a body lumen whose diameter changes dynamically over time where it is often desired to place a stent. Endovascular grafts are composed of a stent or stent-like scaffolding integrated with graft material. Inaccurate graft sizing and the inability of a graft to dynamically adjust to changes in the aorta's diameter during systole and diastole can result in the passage of fluid in the aneurysm between the vessel wall and the graft (endo-leaks), which can result in aneurysm expansion and rupture. An endo-leak is caused by incomplete exclusion of the aneurysm from the arterial circulation by the stent. Endo-leaks are a common complication, occurring in as many as 45% of patients undergoing stent-graft for Abdominal Aortic Aneurysm (AAA) repair.
The lumen of a peristaltic organ is yet another example of a body lumen whose diameter changes dynamically over time where it is often desired to place a stent. However, the presence of a stent in a peristaltic organ may interfer with the propagation of the peristaltic waves. In such cases, peristaltic waves are unable to traverse the stent, thus preventing peristalsis downstream to the stent.